Effect Of Composition And Loading Of Pt Species On The Catalytic Performance Of Pt/CeO₂ Catalyst For CO Oxidation

To explore the structure-activity relationship of CO catalytic oxidation for Pt/CeO2 catalyst,conventional CeO2 with different specific surface area,CeO2 nanorods(CeO2(R))and CeO2 nanocubes(CeO2(C))were made as the supports in this thesis.The Pt/CeO2 catalyst with different Pt species and different Pt loadings were prepared by taking the advantages of the difference of electrostatic adsorption properties of Pt complex precursor ions(PtCl62-,PtCl42-,Pt(NH3)42+)on the surface of CeO2.On the basis of systematic characterization of the structure of the bulk phase,the composition,reduction performance and dispersion of Pt species on the surface,and the valence states of Pt and Ce species.In situ infrared spectroscopy,temperature programmed oxidation characterization and catalytic performance evaluation experimental techniques were used.The variation of Pt species on the surface of the catalyst and their relationships with the performance of CO oxidation were characterized in detail.The main results of this thesis are summarized as follows.(1)The Pt species on the surface of Pt/CeO2 catalyst prepared with anionic Pt complexes(PtCl62-,PtCl42-)as precursors are mainly Pt-O-Ce.When the Pt loading capacity exceeds the saturated loading capacity of CeO2 for Pt-O-Ce species,PtOx species are also generated in addition to Pt-O-Ce species.The main Pt species on the surface of Pt/CeO2 catalyst prepared with cationic Pt complex(Pt(NH3)42+)as precursors are PtOx.(2)For the situation of the Pt/CeO2 catalyst with the same Pt loading and the same CeO2 as support.Pt-O-Ce on it was more difficult to be reduced to Pt0 nanoparticles by CO than PtOx,so the light-off temperature of the former is higher than that of the latter in CO catalytic oxidation reaction.After reducing by H2,both Pt-O-Ce and PtOx were converted to Pt0 nanoparticles,but the size of Pt0 generated by the former was much smaller than that of the latter.The light-off temperature of CO catalytic oxidation of reduced catalyst is lower than that of oxidized catalyst.(3)The Pt0 nanoparticles prepared by reducing the Pt/CeO2 catalyst with Pt(NH3)42+ as the precursors in H2 are not easily reoxidized during the reaction process,which shows a high catalytic activity of CO oxidation at low temperature,and the light-off temperature of the reaction decreases gradually with the increase of the Pt loading.For the reduced Pt/CeO2 prepared with PtCl62-or PtCl42-as precursors,the light-off curve of CO catalytic oxidation reaction is related to the Pt loadings.In the case of a low Pt loading,the light-off temperature of the reaction decreases gradually with the increase of Pt loadings.When the amount of Pt reaches or exceeds a certain "threshold" on the support,the light-off temperature of CO catalytic oxidation reaction increases with the increase of the Pt loadings,instead.Pt0 nanoparticles reduced by Pt-O-Ce on the catalyst are easily reoxidized to Pt2-δ-O-Ce in the process of stepwise heating in the reaction atmosphere,which leads to the degradation of the catalytic activity,and the higher the Pt loading is,the easier the Pt0 species are to be reoxidized to pt2-δ-O-Ce.The minimum Pt loading for the formation of Pt2-δ-O-Ce species on Pt/CeO2 prepared by different supports increases with the increase of specific surface area of CeO2.(4)Pt0 species obtained by reducing Pt/CeO2 with PtCl62-or PtCl42-as precursors own smaller particle size and more interfacial sites between Pt0 and CeO2.When the Pt/Ce ratio on the surface of catalyst reaches or exceeds a certain "threshold",the CeO2 can be reduced to a deeper degree through hydrogen spillover on Pt0 during the H2 reduction process,so the CeO2 support can be fully oxidized by Pt species through oxygen spillover in the O2-containing atmosphere at low temperature.In the subsequent heating process,fully oxidized CeO2 can reoxidize Pt0 nanoparticles through the reverse spillover of oxygen,resulting in the formation of Pt2-δ-O-Ce with low CO catalytic oxidation activity.In the situation of Pt0 species with large particle size(such as Pt/CeO2 prepared with Pt(NH3)42+as precursor)or low Pt loadings,the interfacial sites between Pt0 species and the supports are few.and the hydrogen spillover effect during the H2 reduction process is not enough to reduce the CeO2 supports excessively,and the oxidation degree of CeO2 was also low after switching to the O2-containing atmosphere.Therefore,the performance of catalyst will not be deactivated cause it is hard to oxidize those Pt0 species into Pt2-δ-O-Ce by oxygen reverse spillover.